Traditional motor drive systems often incorporate motor current control and associated torque control. The primary purpose of controlling motor current is to indirectly or directly control the motor torque generated on the rotating shaft of the motor.
FIG. 1 shows a conventional AC motor drive system 2 with closed loop current control. Such a system typically includes a motor controller circuit 4 provided with current sense resistors 6 in the motor drive current lines as well as temperature sensors Tj associated with each IGBT of the inverter. The feedback signals from the current sense resistors 6 and the temperature sensors Tj, together with a torque producing current reference signal Iq*, are input to a current control drive system 8, which includes the conventional components shown in FIG. 1. Based on these inputs, current control drive system 8 produces an appropriate PWM signal to the IGBT gate drive to control the torque of the motor.
Typically, in the event of motor drive fault due to a switching device overtemperature condition, the current control system 8 is designed to automatically shut down the entire motor drive system 2. Complete shutdown protects the motor drive system 2 from catastrophe. However, unexpected shutdown of the entire system is undesirable, disruptive and costly since it involves shutting down not only the motor drive inverter but also coordinating the shutdown of associated machinery.
The present invention advantageously allows the motor drive system to continue operation and avoid an undesirable shutdown of the motor in the event of an overheating condition of the switching devices, e.g., IGBTs, of the inverter. Thus, for example, in a CNC milling machine application, the cutting operation can continue with reduced torque, which allows the switching device temperature to fall to an acceptable level, at which point full torque cutting can be resumed.
The present invention permits uninterrupted operation of a motor, even in the event of an over temperature condition, by dynamically controlling the torque of a motor based on the temperature of switching devices in the motor controller circuit. The circuit of the present invention generates a torque producing signal representative of the difference between the actual torque current and the desired torque current. The circuit limits this torque producing signal to either a fixed limit value if the temperature of the switching devices is below a predetermined value, or to a variable limit value, which is inversely proportional to the temperature signal, if the temperature of the switching devices exceeds the predetermined value. Accordingly, in the event of an over temperature condition, the motor continues to operate, but at a reduced torque, allowing the switching devices to cool.
Other features and advantages of the present invention will become apparent when the following description is read in conjunction with the accompanying drawings.